The present invention relates to a method for manufacturing an oxide superconducting wire in which an oxide superconducting material is coated with silver or an alloy thereof to provide a linear body which is shaped into a tape-shaped wire material. In particular, it relates to a manufacturing method of a superconducting wire having a large critical current density Jc at a service temperature.
A known manufacturing method for such wires is the silver sheath method in which oxides and carbonate salts of starting materials are mixed in a predetermined composition ratio, and repeatedly calcined and ground. The thus treated material is introduced into a silver pipe or tube which is subjected to diameter reduction by means of cold processing, such as, swaging, roll die rolling, groove roll rolling, die drawing and the like. An ordinary rolling is then carried out with a pair of equal speed rolls or a press to control the microstructure, followed by heat treatment (Kozo Osamura and Sang-Soo Oh: Journal of the Japan Society of Powder and Powder Metallurgy, Vol. 38 (1991, 2) 97-102).
In the linear body produced by this silver sheath method, the silver as the outer coat undergoes a plastic deformation, whereas the oxide layer, i.e., the ceramics, which is the core material, does not undergo such plastic deformation. The ceramic has a density which is much lower than that of the silver. This makes it very difficult to uniformly control of the microstructure of the core material after the diameter reduction of the linear body in the longitudinal direction.
For example, when the linear body is subjected to diameter reduction followed by ordinary rolling, i.e., with rolls of equal circumferential speed, to give a tape-shaped wire material, a waving phenomenon occurs in the interior of the body, as shown in FIG. 3, and the thickness of the oxide layer tends to become extremely heterogeneous. As a result, the structure constituting the oxide layer suffers a three-dimensional deformation to form localized voids and the c-axis thereof does not give orientation perpendicular to the tape surface. As is known, "c-axis" is the vertically oriented crystal axis and shows a line normal to the rolled face along which the superconducting current flows. This is a problem since the critical current density Jc cannot be increased.
When the tape-shaped wire material is produced by means of a press instead of the ordinary rolling procedure, these problems are solved. The pressing treatment is not continuous since only a finite area of the tube can be pressed at a given time. The tube must then be moved and the next successive area pressed. However, the linear body must be moved for each press treatment, lowering the productivity. Also, the oxide layer tends to become heterogeneous at the boundary region between each successive press treatment. This causes the problem that a high critical current density (Jc) cannot be obtained when the wire material becomes longer.